A multilayer ceramic capacitor comprises dielectric ceramic layers, internal electrodes disposed between the dielectric ceramic layers, and an external electrode connected to these internal electrodes on both sides of the dielectric ceramic layers.
Ceramic compositions having a high dielectric constant comprising barium titanate as a major component have conventionally been used as a material for dielectric ceramic layers. In particular, a composition comprising barium titanate as a major component and, incorporated therein as a minor component, a bismuth compound, such as the titanate, stannate, and zirconate of bismuth oxide, has been widely employed from the standpoint of diminishing fluctuations of dielectric constant with temperature and voltage. The conventional dielectric ceramic layers have been formed by firing such a dielectric material at temperatures around 1,200.degree. C.
Because of such high temperatures used for the firing of dielectric materials, substances having a high melting point and less susceptible to oxidation at high temperatures, e.g., a silver-palladium alloy and platinum, have been used as a material for internal electrodes. After the formation of dielectric ceramic layers and internal electrodes, external electrodes have been formed by baking silver, etc.
However, the use of noble metals such as platinum and silver-palladium alloys as internal electrodes has been a serious obstacle to cost reduction in multilayer ceramic capacitors, since these materials are expensive. Another problem is that the internal electrodes made of a silver-palladium alloy may suffer deterioration in properties due to silver migration. Furthermore, the internal electrodes made of platinum have a drawback that the electrodes have an increased equivalent series resistance because of the low electrical conductivity of platinum.
It has been proposed that a base metal having a high melting point, such as nickel, cobalt, and tungsten, may be used for overcoming the above-described problems. However, these base metals are so readily oxidized in a high-temperature oxidizing atmosphere that they do not function as an electrode. Therefore, for using these base materials as the internal electrodes of multilayer ceramic capacitors, it is necessary that they must be fired in a neutral or reducing atmosphere along with a dielectric material.
However, if the dielectric material described above comprising barium titanate as a major component and a bismuth compound as a minor component is fired in a neutral or reducing atmosphere, the barium titanate and bismuth oxide contained in the dielectric material are reduced and this results in a problem in that the dielectric ceramic layers thus obtained have a reduced insulation resistance.